This application relates to the recovery of liquid and gaseous products from oil shale. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the carbonaceous liquid product is called "shale oil".
One technique for recovering shale oil is to form an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort being formed is explosively expanded to form a fragmented permeable mass of particles containing oil shale.
The fragmented mass is ignited near the top to establish a combustion zone. An oxygen containing gas is introduced in the top of the retort to sustain the combustion zone and cause it to advance downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds the heat of combustion is transferred to the fragmented permeable mass of particles containing oil shale below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. Thus, a retorting zone advances from top to bottom of the retort in advance of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal.
In preparation for the retorting process it is important that the formation containing oil shale be fragmented rather than simply fractured to create sufficient permeability that undue pressures are not required to pass the gases through the retort. The aforementioned patent applications provide techniques for fragmenting a substantial volume of formation containing oil shale to form a fragmented permeable mass in an in situ oil shale retort. In these applications an in situ oil shale retort is formed in a subterranean formation containing oil shale by excavating a void having a vertically extending free face, drilling blasting holes adjacent to the columnar void and parallel to the free face, loading explosive into the blasting holes, and detonating the explosive to expand the formation adjacent the columnar void toward the free face. In one embodiment the void is cylindrical and blasting holes are arranged in concentric rings around the void. In another embodiment the void is a slot having one or more large parallel planar vertical free faces toward which the formation in the retort volume is explosively expanded. In such an embodiment the blasting holes are preferably arranged parallel to the free faces. For larger retorts a plurality of voids can be excavated and the formation expanded toward the respective voids to form a continuous fragmented permeable mass of particles containing oil shale.
In applications Ser. Nos. 659,899 and 658,699, a plurality of vertically spaced apart voids providing horizontally extending free faces are excavated in the retort site and remaining formation in the retort site is explosively expanded toward such voids for forming a fragmented permeable mass.
In embodiments disclosed in application Ser. No. 603,704 a base of operation is excavated in formation at the site of an in situ retort to be formed. An access tunnel is excavated to a location below the base of operation. A columnar void is excavated in communication with the lower tunnel. The top of the columnar void is spaced from the bottom of the base of operation at the top to leave a horizontal pillar of intact formation between the top of the columnar void and the bottom of the base of operation. This leaves the floor of the base of operation free from a hazardous condition, namely, a large opening during operations conducted therefrom. The debris or muck created during formation of the columnar void can be drawn from the lower tunnel for removal.
It has been found that it can be desirable to have an intact subterranean base of operation above a fragmented permeable mass of particles in an in situ oil shale retort. Such a base of operation facilitates ignition over the top portion of the fragmented mass, permits control of introduction of oxygen containing gas into the retort, provides a location for testing properties of the fragmented mass such as distribution of void fraction and for evaluating performance of the retort during operation, and aids in controlling flow of ground water which might otherwise enter the fragmented mass during retorting.